The design of engines that travel at supersonic speeds (e.g. possibly in the range of Mach 2 to Mach 3 at cruise, or possibly higher), involves a number of problems similar to those encountered in the design of sub-sonic jet engines. Thus, there are with both supersonic and sub-sonic engines the general concerns of weight, size, complexity, reliability, cost, etc., and also concerns relative to performance (e.g. thrust, specific fuel consumption, etc.). However, supersonic jet engines pose some special problems. For example, present day optimized designs for supersonic turbojet type engines are characterized in that these have relatively high jet velocities, and also create a high level of noise. Noise suppression in this type of engine is one of the most critical technical problems to be solved in making an environmentally acceptable commercial supersonic jet transport. Another consideration is that a supersonic jet engine must be designed to function adequately through a broad range of operating modes (i.e. take off and climb, acceleration up to supersonic cruise Mach number, as well as being able to cruise at both subsonic and supersonic speeds). Further, the general concerns relating to both subsonic and supersonic engine are exacerbated by the more stringent performance requirements imposed on supersonic jet engines.
With regard to noise suppression, over the last several decades, there have been many different systems proposed and/or used for suppressing noise. One general approach has been to mix the higher velocity jet exhaust with lower velocity air, and there are innumerable patents and other technical disclosures relating to variations on this basic concept. However, quite often these mixing type noise suppressors will degrade performance. This has been particularly true with supersonic engines. One approach to solve this problem has been to deploy the sound suppressing apparatus in its sound suppressing functioning mode during takeoff and climb and other situations where sound suppression apparatus is needed, and then to provide means by which the noise suppressing apparatus could be "stowed" for other operating modes (e.g. supersonic cruise). However, this takes extra space and adds complexities.
Further, the trend in subsonic turbofan engines has been to build engines with relatively large by-pass ratios so that most of the energy developed by the engine is actually transmitted into the fan. Thus jet noise is lowered and suppression is not required as much. However, in supersonic turbojet engines, when a fan is incorporated in the design, the by-pass ratio is generally quite small (e.g. 0.03 to 0.1), and the fan air is often able to provide not too much more than a cooling function. In some instances, it has been proposed to mix the fan air in a supersonic jet engine with the jet exhaust, and this is in some designs incorporated with an after burner. To the best knowledge of the applicant, this mixing would normally occur while both the fan air and the jet exhaust are both subsonic.
In recent years, there have been discussions in the technical literature on ejectors where there is supersonic mixing. For example, in Volume 21, Number 10 of the AIA Journal, there is an article "Thrust Augmenting Ejectors, Part I", written by Morton Alperin and Jiunn-Jen Wu. A second article appeared in Volume 21, Number 12 of the AIA Journal, bearing the title "Thrust Augmenting Ejectors, Part II", by these same authors. There is an analysis of compressible fluids through a thrust augmenting ejector, and these articles deal with two distinct flows after substantially complete mixing has been accomplished. There is the "first solution", where there is subsonic mixed flow and the "second solution", where there is a supersonic mixed flow. Also there is a later publication which is "NASA Contractor Report 177419", which is authored by Dr. Wu, this being prepared for the Ames Research Center in July, 1986, and both first and second solution ejectors and the tests conducted on these are discussed. Also, there is a publication "Compound-compressible Nozzle Flow", authored by Arthur Bernstein, William Heiser and Charles Havenor, presented at the AIAA 2nd Propulsion Joint Specialist Conference at Colorado Springs, Colo., Jun. 13-17, 1966, and this deals with the behavior of two or more gas streams flowing through a single nozzle.